In the electronics industry, pneumatically actuated “jetting” dispensers are commonly used to selectively dispense small amounts or droplets of a highly viscous material in a non-contact manner onto a substrate or electronic package. Exemplary highly viscous materials include, but are not limited to, solder flux, adhesives, solder paste, solder mask, thermal compounds, oil, encapsulants, potting compounds, inks, conformal coatings, and silicones. Generally, such highly viscous materials cannot easily flow under their own weight at room temperature.
A typical jetting dispenser apparatus includes an air-operated valve element or needle configured to selectively engage a valve seat surrounding a discharge passage. The contact between the valve element and valve seat seals off the discharge passage from a chamber supplied with viscous material under pressure. Thus, to dispense droplets of the viscous material, the valve element is retracted from contact with the valve seat to allow an amount of the viscous material to flow through the newly formed gap and into the discharge passage. The valve element is then moved rapidly toward the valve seat to close the gap, which forces the amount of viscous material through the discharge passage and causes a droplet of the material to be ejected, or “jetted,” from an outlet of the discharge passage. The droplet eventually lands on a substrate spaced apart from the dispenser outlet.
Because jetting dispensers do not need to be repositioned in a z-axis direction every time a droplet of the viscous material is ejected, the cycle time between droplets is significantly reduced when compared to other dispensers. Unlike conventional needle dispensers, which rely upon contact between a needle and a surface, jetting dispensers are able to “fly” above the substrate at a fixed height and “jet” the material onto an intended application area without any contact. By rapidly jetting the material “on the fly” (i.e., while the dispenser is in motion), the dispensed droplets may be joined to form a continuous line. Jetting dispensers may therefore be easily programmed to dispense desired patterns of viscous material. This versatility has made jetting dispensers suitable for a wide variety of applications in the microelectronics industry.
For example, jetting dispensers are commonly used to selectively apply solder flux in flip-chip applications. Flip chip assembly typically involves mounting an electronic component, such as a semiconductor die or chip, onto a substrate, such as a printed circuit board. After applying flux to a desired area on the substrate, the chip is mounted such that an active surface having solder “balls” or “bumps” is flipped over and placed in registration with electrical bond pads on the substrate. The solder is then reflowed to create electrical and mechanical interconnections in the form of solder joints extending between the chip and the packaging substrate. During the beginning of the reflow process, the solder flux removes oxide and other surface films of contamination from the substrate and promotes the flowing of the molten solder to form the solder joints.
Frequently, the component to be mounted to the substrate includes an array of solder bumps. To quickly and effectively coat the corresponding bond pad or area on the substrate with solder flux, the jetting dispenser may be modified to apply a thin film or layer of the flux. In particular, the jetting dispenser may be equipped with a nozzle having a coaxial air discharge orifice with the outlet of the discharge passage. The nozzle directs pressurized air through the air discharge orifice to atomize the jetted droplets of flux into smaller particles and to spread the flux into a thin layer on the substrate. Jetting dispensers incorporating such a co-axial air option are therefore capable of covering desired areas with a minimal amount of flux.
Although applying a thin layer of flux in this manner may offer several significant advantages over conventional dipping and screen printing techniques for applying flux, there remain several challenges associated with do so. For example, it can be difficult to obtain good edge definition when applying a layer of solder flux with a jetting dispenser. This challenge is largely a result of the size of the components to which flux is to be applied, the size of the surrounding areas to which flux is not to be applied, the speed of jetting dispensers, and the viscous nature of the flux material.
An example of poor edge definition is generally shown in FIG. 1, which illustrates a printed circuit board 10 after a layer of solder flux 12 has been applied with a jetting dispenser (not shown). The circuit board 10 includes a bond pad 14 with a bump field configured to register with solder bumps (not shown) on an associated chip or other component. Although the area of bond pad 14 is effectively covered with flux 12, overspray in the form of “spider tails” and other errant strands 16 extend beyond a perimeter 18 of the bond pad 14. The errant strands 16 may be a result of atomized particles of flux that rebound outwardly after contacting the circuit board 10 at an angle, amounts of flux being spread across the surface of the circuit board 10 by pressurized air, or a combination of the two. In particular, pressurized air directed around the outlet of the jetting dispenser and the high velocity of droplets ejected from the jetting dispenser may cause some uncontrolled migration as the flux 12 contacts the circuit board 10. This migration makes the final dispensed shape of the flux 12 difficult to control or predict.
The strands 16 may interfere with or contaminate other features or components on the circuit board 10 near the bond pad 14. As a result, secondary cleaning processes may be necessary to remove the overspray or masks may need to be applied to the circuit board 10 before dispensing the flux 12. Both of these options are time-consuming and decrease the overall throughput of the dispensing system. The concerns for poor edge definition may also prompt circuit board and chip designers to arrange components in a manner that sacrifices overall electronic packaging density on the printed circuit board 10.
Therefore, an improved method of applying viscous materials with a jetting dispenser is needed. The method should effectively coat desired areas on a substrate in a manner that reduces the amount of overspray or errant material associated with poor edge definition.